Connecticut senators Richard Blumenthal and Chris Murphy tweeted after the horrific shooting.
"Another nightmare -- so sickening, savage, stunning, but utterly unsurprising. Congress' inaction on gun violence makes this horror so tragically predictable," Sen. Blumenthal said.
"This is the moment to make our stand. Now," Sen. Murphy said in a Tweet.
"No more Newtowns. No more Parklands. No more Boulders. Now - we make our stand," he added in part.
The Newtown Action Alliance also sent a Tweet telling people to call Congress and urge them to ban assault weapons and other gun control laws.
The first 911 calls started coming in to the Boulder Police Department around 2:30 in the afternoon that there was a gunman at the King Soopers grocery store.
The first officer to respond was 51-year-old Eric Talley. He had been with the police department since 2021. He died, along with nine other people in the store.
Boulder police said the gunman was wounded during the incident and was taken into custody.
"From my perspective, there is more for us to do than just give them our thoughts and prayers. We are gonna go all out to ensure that the right result is reached. And that's why I’m very grateful that we do have local state and federal agencies all responding to this location here today. To ensure that the investigation is thorough and complete, and ensures that we reach justice in this case," Boulder District Attorney Michael Dougherty said.
The investigation is still in the early stages and police say they do not have a motive.
The attack in Boulder was the seventh mass killing this year in the U.S., following the March 16 shooting that left eight people dead at three Atlanta-area spa businesses, according to a database compiled by The Associated Press, USA Today and Northeastern University.